Various fluid dispensers include movable valve elements, such as valve stems, for allowing and preventing fluid flow from the dispenser. Electromagnetically actuated dispensing apparatus, also known as electric guns, include a magnetic pole piece, a magnetic plunger or armature, a valve stem coupled for movement with the armature and an electromagnetic coil which is part of the actuation means for moving the plunger. The armature can be moved relative to the pole piece by selectively energizing and de-energizing the electromagnetic coil. The movement of the plunger or armature towards the pole piece disengages the valve stem from a valve seat and opens the dispensing module. When the electromagnetic coil is de-energized, a biasing means, preferably a return spring, forces the armature away from the pole piece. The valve stem is urged into contact with the valve seat to close the module. Such modules are known from U.S. Pat. No. 6,994,234 or EP 0908240, owned by the present applicant.
The frequency of the reciprocating movement of the plunger is referred to as cycle rate. The cycle rate has an influence on the shape of the material dispensed on a substrate. In order to improve the cycle rate, at least one flux guide element is conventionally employed in order to strengthen the magnetic field by improving the magnetic flux. These flux guide elements can be tubular structures surrounding the plunger, the electromagnetic coil and/or the pole piece and can be formed of the housing. During operation, eddy currents can be generated within the flux guide elements, which can decrease the cycle rate. Circumferential electrical currents retard the dissipation of the magnetic field. In order to reduce such eddy currents, U.S. Pat. No. 6,994,234 suggests a gap in a flux guide element which is formed by the housing.
However, there is a need to further improve the performance characteristics, in particular the cycle rate of a fluid dispensing apparatus capable of operating and high frequencies. Thus, it is the object of the invention to provide an improved liquid dispensing apparatus.